Mad Cow Disease and Variant Creutzfeldt-Jakob Disease

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Mad Cow Disease and Variant Creutzfeldt-Jakob Disease

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Mad Cow Disease Overview

“Mad cow” is an infectious disease in the brain of cattle. Humans who become infected, usually by eating tissue from diseased cattle, will die of a similar brain disease that may develop over many years.

Abnormal proteins called prions (PRE-ons) are found in brain tissue of diseased cattle. Prions eat away at the brain and create tiny spongelike holes in parts of the brain. These so-called spongy holes cause slow deterioration within the cattle brain, and eventually symptoms affecting the whole body. Death follows. The scientific name for mad cow disease in cattle is called bovine spongiform encephalopathy (meaning sick brain) or BSE, meaning a sickness of the cow’s brain; when damaged brain tissue is viewed on a laboratory slide, it has a spongy appearance.

If humans eat diseased tissue from cattle, they may develop the human form of mad cow disease known as variant Creutzfeldt-Jakob disease (vCJD) or new variant Creutzfeldt-Jakob disease (nvCJD). The disease was named after the researchers who first identified the classic condition. Creutzfeldt-Jakob disease in its classic form usually occurs in older people through an inherited tendency of the brain to change or spontaneously for no apparent reason. The type identified as occurring from eating diseased cattle occurs in younger people and has atypical clinical features, with prominent psychiatric or sensory symptoms at the time of clinical presentation and delayed onset of neurologic abnormalities. These neurologic abnormalities include ataxia within weeks or months, dementia (loss of memory and confusion) and myoclonus late in the illness, a duration of illness of at least 6 months, and a diffusely abnormal non-diagnostic electroencephalogram.

The transmissible agents that cause the disease in both cattle and humans are “prions.” Prions are not like bacteria or viruses that cause other infectious diseases; rather, they are infectious proteins.

Diseased prions are found in the brain, spinal cord, eye (in the retina), and other tissues of the nervous system of affected animals or humans. In addition, prions can be found outside the nervous system including the bone marrow, spleen, and lymph nodes. Low levels of prions may also be found in blood.

Prions are highly resistant to heat, ultraviolet light, radiation, and disinfectants that normally kill viruses and bacteria. Prions may infect humans who eat meat from infected cattle. Even cooking meat infected with BSE does not eliminate the prions or the risk.

Once infection occurs, there is a long incubation period that typically lasts several years. When prions reach a critical level in the brain, symptoms such as depression, difficulty walking, and dementia occur and progress rapidly.

Scientists believe that BSE is transmitted from animals to humans when humans eat meat from infected animals. The content of infected brain tissue may be higher in some food products than others, and it may also depend on the way the animal was slaughtered.

BSE be transmitted from one human to another through cannibalism or through transplantation of infected tissue. Consequently, certain human blood products and blood donations are not accepted from people who have lived in areas of the world where BSE outbreaks have occurred in cattle. BSE has been shown to be transmissible by blood transfusion in an experimental model using sheep. The blood of BSE-infected sheep has been shown to be infectious even in the preclinical stage of disease, and there has been a report of possible transmission of vCJD by blood transfusion to a single person in the United Kingdom.

In December 2003, the first case of BSE in the United States was detected in a dairy cow in the state of Washington. Before that, a devastating, major outbreak occurred in the United Kingdom (England and Ireland) in the 1980s. BSE and resulting cases of vCJD in humans have been diagnosed in the UK and in European countries such as Bosnia-Herzegovina, Liechtenstein, Macedonia, Norway, Sweden, and Yugoslavia. Because there is no way to detect BSE in blood, people who have lived in these areas from 1980 to the present are not allowed to donate blood in the US, under January 2002 guidelines from the US Food and Drug Administration.

Background

In the mid-1980s, an epidemic of BSE was seen among cattle in the United Kingdom. Because the suspected cause was a prion transmitted in meat and bone meal products fed to cattle there, the government banned the practice of feeding such products that may contain diseased tissue to animals in 1988. By then, however, infected cattle had already entered the human food supply.

By 1996, several people in the UK and others who had lived there were identified with a variant form of CJD, and the cause was linked to eating meat from cattle infected with mad cow disease. As of December 2003, 143 people with vCJD had been diagnosed in the UK.

Prion disease overview

Overall, prion diseases are a large group of related conditions affecting the nervous system, which affect both animals and humans. Included is Creutzfeldt-Jakob disease (CJD), discussed in detail here in relation to bovine spongiform encephalopathy (BSE, mad cow disease). Another human prion disease is Gerstmann-Sträussler-Scheinker (GSS).

In animals, chronic wasting disease (CWD) is found in mule deer and elk in the US, and scrapie is a similar condition found in sheep. Cases have been reported in the US. These diseases all take a long time to develop but are typically rapidly progressive once symptoms begin.

All prion diseases are fatal. Animals and humans who develop a prion disease will die of it. There is no effective treatment. It is important to understand how these diseases are transmitted in order to prevent their spread.